12.3 Origin of Life
KEY CONCEPT
The origin of life on Earth remains a puzzle.
12.3 Origin of Life
Earth was very different billions of years ago.
• There have been many hypotheses of Earth’s origins.
• The most widely accepted hypothesis of Earth’s origins is
the nebula hypothesis.
12.3 Origin of Life
Several sets of hypotheses propose how life began on
Earth.
• There are two organic molecule hypotheses.
– Miller-Urey experiment: created organic molecules using
electrical current (lightning)
electrodes
heat source amino acids
water
“atmosphere”
“ocean”
12.3 Origin of Life
Several sets of hypotheses propose how life began on
Earth.
• Meteorite hypothesis:
– Organic molecules arrived from outer space
12.3 Origin of Life
• There are different hypotheses of early cell structure.
– iron-sulfide bubbles hypothesis:
– hydrothermal vents make sulfur + ocean water =
conditions for life
12.3 Origin of Life
• There are different hypotheses of early cell structure.
– lipid membrane
hypothesis:
– lipid molecules
spontaneously form
membrane-enclosed
spheres
12.3 Origin of Life
• A hypothesis proposes that RNA was the first genetic
material.
12.3 Origin of Life
12.4 KEY CONCEPT
Single-celled organisms existed 3.8 billion years ago.
12.3 Origin of Life
Microbes have changed the physical and chemical
composition of Earth.
• The oldest known fossils are a group of marine
cyanobacteria.
– prokaryotic cells
– added oxygen to
atmosphere
– deposited minerals
12.3 Origin of Life
• Fossil stromatolites provide evidence of early colonies of
life.
12.3 Origin of Life
Eukaryotic cells may have evolved through
endosymbiosis.
• Endosymbiosis is a relationship in which one organism lives
within the body of another.
• Mitochondria and chloroplasts may have developed through
endosymbiosis.
12.3 Origin of Life
The evolution of sexual reproduction led to increased
diversity.
• Genetic variation is an advantage of sexual reproduction.
• Sexual reproduction may have led to the evolution of
multicellular life.
12.3 Origin of Life
12.6 KEY CONCEPT
Humans appeared late in Earth’s history.
12.3 Origin of Life
Humans share a common ancestor with other primates.
• Primates are mammals with flexible hands and feet,
forward-looking eyes and enlarged brains.
12.3 Origin of Life
• Primates evolved into prosimians and anthropoids.
– Prosimians are the oldest living primates.
– They are mostly small and nocturnal.
12.3 Origin of Life
– They are subdivided into the New World monkeys, Old
World monkeys, and hominoids.
– Anthropoids are humanlike primates.
– Homonoids are
divided into
hominids, great
apes, and lesser
apes.
– Hominids include
living and extinct
humans.
12.3 Origin of Life
• Bipedal means walking on two legs.
– foraging
– carrying infants and food
– using tools
• Walking upright has
important adaptive
advantages.
12.3 Origin of Life
There are many fossils of extinct hominids.
• Most hominids are either the genus Australopithecus or
Homo.
• Australopithecines were a successful genus.
• The Homo genus first evolved 2.4 million years ago.
12.3 Origin of Life
Modern humans arose about 200,000 years ago.
• Homo sapiens fossils date to 200,000 years ago.
• Human evolution is influenced by a tool-based culture.
• There is a trend toward increased brain size in hominids.
Australopithecus
afarensis
Homo habilis Homo
neanderthalensis
Homo sapiens